Environmental concerns and governmental regulations require reduced emissions of volatile hydrocarbon fuel vapors into the atmosphere. One source of hydrocarbon fuel vapors is the fuel tanks of vehicles using gasoline or other hydrocarbon fuels with high volatility. Fuel vapor can escape to the atmosphere during the filling of the tanks and usually even after the tanks are filled.
The use of an onboard vapor recovery system to remove excess fuel vapor from the fuel tank is one solution to the problem. Typically, a canister with activated charcoal therein receives fuel vapors through a vent valve mounted in the top of the fuel tank or within a flange of an in-tank fuel pump module for communication with a vapor dome in the tank. The vent valve is usually responsive to the level of fuel in the tank and will generally stay open provided the fuel level within the tank is sufficiently low. When open, fuel vapors flow freely from the fuel tank into the canister. Some vent valves are referred to as fill-limit vent valves or FLVV, because when the fuel tank of the vehicle is being refueled by a automatic shut-off fuel pump nozzle, the fuel level rises until a predetermined maximum level is reached. This maximum level generally preserves a minimum size vapor dome above the fuel.
For refueling purposes of the fuel tank, a filler tube generally extends substantially downward to the tank and communicates directly with the tank at an opening. When following common government regulatory requirements that a vehicle must generally sit within about a plus or minus three degree angle to a horizontal plane, the filler tube opening at the tank is commonly located above the maximum fuel level and communicates with the minimum size vapor dome. This relationship assures that when nearing maximum fuel level and before the FLVV closes, a backpressure is not created in the filler tube at the opening, because such a backpressure would cause liquid fuel to gurgle or backup in the filler tube. Such a backup could cause the automatic shut-off fuel pump nozzle to prematurely shut-off before maximum fuel level is reached.
During refueling of the vehicle and as the fuel level rises to a predetermined maximum level, a float of the vent valve rises with the fuel level to close the valve thus preventing liquid fuel from flowing through the vent valve and into the vapor receiving canister. Two such vent valves are disclosed in U.S. Pat. Nos. 6,145,532 and 6,848,463, and incorporated herein by reference in their entirety.
Known vapor vent valves are typically mounted rigidly to the fuel tank at substantially the highest elevation to vent away most of the fuel vapor to the canister during refueling when the tank or vehicle is generally at a horizontal position to thereby control the minimum volume of the vapor dome. Regardless of whether the combustion engine is running, the open vent valves allow air and fuel vapor, but not liquid fuel, to flow from the tank and to the canister. When the combustion engine is running and fuel is being displaced from the tank, a one-way venting check valve preferably vents fresh air to the enlarging vapor dome in the tank while air and fuel vapor may continue to flow through the open vent valve(s), then through the canister and to the running engine to maintain substantially constant pressure in the fuel tank.
Unfortunately, if the tank has two vapor domes or two high elevation points, known fuel storage systems having only one fixed vent valve can vent only one of the vapor domes. Because the vapor in the other vapor dome can not be displaced with fuel, the storage capacity of the tank is undesirably limited. Moreover, if the vehicle is traveling down or up a steep embankment, the tank is no longer generally horizontal and a substantially full tank of fuel could submerge the float of the fixed vent valve thus closing the vent valve while the engine is operating. With the vent valve closed and the engine consuming fuel or with the fuel being heated by a return loop fuel system, constant internal pressure of the tank is disrupted and engine performance may be degraded.
Moreover, for off-road vehicle applications that require gravity fed manual refueling operations (i.e. from a portable five gallon gas can), the vehicle may not be sitting within a plus or minus three degree angle from a horizontal plane as required for automatic shut-off pump refueling operations. Instead, the vehicle could be tilted at a much greater angle causing the FLVV to close considerably before the vapor dome is reduced to a minimum volume. Although premature closure of the FLVV on its own may not pose a filler tube backup problem during a manual refueling operation because supply fuel typically flows through the filler tube at a much slower rate, trapped air and fuel vapor in the tank can greatly reduce it's liquid fuel storage capacity when the tank is orientated at excessive angles away from the horizontal plane. That is, with the FLVV closed, and once the filler tube opening at the tank is immersed in liquid fuel, and air and vapor remaining in the tank is trapped. The volume of this trapped air and vapor may greatly exceed the minimum required volume of the vapor dome.